Certainly, the time difference is noticable, but our DPS will still be higher per two casts. Remember, you're still waiting by the time the second Fire IV hits, so it's 540/570 potency every 4.17 vs 982 potency every 5.46.

As for Thunder III's randomness, yes, it's inconsistent due to RNG, but that makes up for its massive power. And Sharpcast T3 is only as situational as having a guaranteed proc every 60s is. I mean, it's not like Verstone and Verfire lack their own (albeit, more guaranteed) procs. Saying Thunder III has no place in discussion is malarkey due to its necessity as a second burst in the BLM's arsenal, one that can be repeatedly spammed when obtained, and is only balanced by the fact that it's RNG-based.